THE BLASTOPOEE. Ill 



the yolk-plug being separated from each other by the narrow 

 circular groove which leads into the mesenteron. In the im- 

 mediately succeeding stages the blastopore becomes greatly 

 reduced in size, though still retaining its circular outline 

 (cf. Figs. 52, 54, 55). This reduction is effected, not by con- 

 traction of the whole circumference of the blastopore, but by 

 a folding together, or concrescence, of its lips in the median 

 plane, beginning at the lower or ventral margin and proceeding 

 upwards towards the dorsal margin, the line of fusion being 

 marked by a faint vertical groove on the surface of the embryo 

 (cf. Fig. 58, A and B). 



At the lip of the blastopore, round its entire circumference, 

 the three germinal layers, epiblast, mesoblast, and hypoblast, 

 are indistinguishably fused together (Figs. 54, 55) ; the sepa- 

 ration between the layers first appearing a little distance beyond 

 the margin of the blastopore. As the lips of the blastopore 

 meet and unite from below upwards, in the manner described 

 above, a vertical band is produced by their union, at the hinder 

 end of the embryo, in which the three germinal layers are fused. 

 This band is spoken of as the primitive streak ; and the faint 

 median groove, already described (Fig. 58, B, C), which runs 

 along it, and marks the line of union of the right and left lips 

 of the blastopore, is named the primitive groove. 



The primitive streak and primitive groove are comparatively 

 inconspicuous features in the frog embryo, but are much more 

 prominent in the chick and the rabbit. They are probably 

 to be regarded as secondary rather than as essential characters, 

 and as associated with the great distension which the egg has 

 undergone in consequence of the number of yolk-granules 

 imbedded in its substance. 



The further development of the primitive streak, and the 

 ultimate fate of the blastopore, will be described in a later part 

 of this chapter. 



The reduction in size of the blastopore, caused by the con- 

 crescence of its lips, gives rise to a corresponding diminution 

 of the yolk-plug (cf. Figs. 52, 54, YP) ; and at the close of the 

 period now being described this withdraws completely from the 

 surface of the embryo (Fig. 55). 



